In this video we're gonna learn about the development of the eye. In the third week of development the notochord appears in the mesoderm. The notochord secretes growth factors which stimulate the differentiation of the overlying ectoderm into neuroectoderm forming a thickened structure known as neural plate The lateral edges of the neural plate then rise to form neural folds. The neural folds move towards each other and meet in the midline fusing to form the neural tube which is the precursor.
to the brain and spinal cord. The formation of neural tube is known as neurulation and is achieved by the end of the fourth week of development. In the fifth week of development, Swirlings appear at the cranial end of the neural tube.
Three primitive vesicles appear and these are prosencephalon which is again divided into two secondary vesicles telencephalon and diencephalon and we have mesencephalon and rhombencephalon which again gives metencephalon and myelencephalon as secondary vesicles The area of neural plate which forms the diencephalon develops a linear thickened area on either side. The thickened area soon becomes depressed to form optic sulcus. The distal part of the optic sulcus enlarges to form optic vesicle whereas the proximal area remains as it is or gets constricted to form optic stalk. and of course they are surrounded by the surface ectoderm.
the optic vesicle grows laterally and comes in contact with the surface ectoderm. the surface ectoderm overlying the optic vesicle becomes thickened to form the lens placode. The lens placard thickens further and sinks below to form lens vesicle.
The lens vesicle finally detaches from the surface ectoderm at 33rd day of gestation and invaginates into the optic vesicle. Due to invagination by the lens the optic vesicle gets converted into cop shaped double layered structure called optic cop. However, the lens is not covered by the optic cup below. This deficiency extends to some distance along the inferior surface of the optic stalk and is called the coroidal fissure or fetal fissure.
The developing neural tube is surrounded by mesenchyme. An extension of this mesenchyme also covers the optic vesicle. The mesenchyme has two layers outer fibrous layer and inner vascular layer. In the brain the outer fibrous layer gives rise to durameter and the inner vascular layer forms arachnoid meter and piameter.
However these layers covering the optic cup forms the structures of the eye. the outer fibrous layer colored in blue along with the surface ectoderm forms cornea of the eye we know cornea have different layers outer epithelial layer followed by Bowman's layer stroma predesmet's membrane desmet's membrane and endothelium all these layers arise from fibrous mesenchyme except the epithelial layer which arises from surface ectoderm. posteriorly the fibrous mesenchyme forms sclera and extraocular muscles. the inner vascular mesenchyme forms choroid and ciliary body anteriorly.
Now on to the development of retina. We know retina has two layers outer pigmented layer and inner neural layer. The pigmented layer is derived from the outer wall of the optic cup colored in red hair. Cells of the outer wall of the optic cup become pigmented. Its posterior part forms the pigmented epithelium of retina and the anterior part continues forward in ciliary body and iris as their anterior pigmented epithelium.
The neural part of retina is derived from the inner wall of optic cup colored in orange. Initially the inner wall of the optic cup is a single layer epithelium. It divides into several layers of cells which differentiate into the following three layers Matrix, Mantle and Marginal. Cells of matrix layer form the rods and cones which are the photoreceptors of the eye.
Cells of mantle layer form bipolar cells. ganglion cells and supporting tissues. Cells of marginal layer also form ganglion cells and their nerve fibers which ultimately form optic nerve. The vascular mesenchyme trapped inside the optic cup forms halloid system which temporarily nourishes the embryonic lens but disappears later as the vitreous proper starts to develop. I said earlier that ciliary body is formed from vascular mesenchyme.
However, it's not completely true. The vascular mesenchyme form stroma of ciliary body, ciliary muscles and blood vessels. The two layers of the optic cup forms the epithelium of the ciliary body. Same goes for the iris. The stroma and blood vessels are formed from vascular mesenchyme and the epithelium are formed from the optic cup.
The sphincter pupilli and dilator pupilli muscles in the iris are also formed from the neuroectoderm of optic cup. Eyelids and conjunctiva develop from the surface ectoderm. Now let's talk about the formation of lens. Lens is developed from lens vesicles and we know how lens vesicle is formed. The developing neural retina signals the cells closest to the posterior end of the lens vesicle to elongate towards the anterior end of the vesicle.
These cells are called primary lens fibers. The signals from neural retina also induces the synthesis of crystallines. Crystalline is a water soluble structural protein found in the lens and the cornea of the eye accounting for the transparency of the structure.
These elongating lens eventually fill in the lumen of the vesicle and they become embryonic nucleus in mature lens. Cells present on the equatorial region of the lens forms a secondary lens fibers. These fibers elongate and covers the primary lens fibers. The lens continues to grow after birth with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium in a region referred to as the germinative zone